Upper and lower plate juxtaposition, deformation and metamorphism during crustal convergence, Trans-Hudson Orogen (Quebec–Baffin segment), Canada

Abstract

The lower plate of the Quebec–Baffin segment of Paleoproterozoic Trans-Hudson Orogen comprises parautochthonous Archean crystalline basement of the Superior Province (3.22–2.74 Ga) and imbricated Paleoproterozoic rift-margin strata (2.04–1.92 Ga). The collisional upper plate contains (1) allochthonous ophiolitic units (2.00 Ga), (2) a continental arc terrane (Narsajuaq arc; 1.86–1.82 Ga), (3) shelf sequence strata (ca. <1.93 Ga and >1.86 Ga) and basement orthogneisses (ca. 1.95 Ga), (4) foreland basin deposits (<1.93 Ga and >1.86 Ga), and (5) a batholith which intrudes the shelf and foreland basin rocks (Cumberland batholith; 1.86–1.85 Ga). Lower and upper plate assemblages were juxtaposed during a 1.82–1.79 Ga accretion–collision event, and deformed into regional-scale folds during two post-collisional folding episodes (ca. 1.76–1.74 Ga). As a result of the structural relief generated by the post-collisional folding and subsequent exhumation, all orogenic elements can be directly examined along a 400 km, across-strike transect. In the external zone (northern Quebec), lower plate rift-margin units were internally imbricated (Cape Smith Belt) and translated southward across the Superior Province basement. Paleoproterozoic amphibolite facies mineral growth is syn to post-thrusting. Tectonic imbrication has resulted in a structural inversion of the accreted (i.e. upper plate) ophiolite stratigraphy. North of the Cape Smith Belt, mid-crustal plutonic units of the Narsajuaq arc form an upper plate crystalline wedge whose sole thrust is localized near the thrust belt's basal décollement. Within Narsajuaq arc, syn-magmatic granulite facies assemblages are retrogressed to amphibolite facies assemblages in proximity to major accretion–collision faults. In the internal zone (southern Baffin Island), a tripartite orogen-scale tectonostratigraphy is exposed on the flanks of the post-collisional fold structures. At the lowest structural level, lower plate Superior Province orthogneisses are imbricated with Paleoproterozoic rift-margin units, and mineral growth is syn-thrusting. At intermediate structural level, upper plate orthogneisses are correlated with the Narsajuaq arc in northern Quebec. At the highest structural level, a north-facing shelf sequence is structurally overlain by an assemblage of clastic rocks interpreted as a foreland basin sequence. Dominantly-monzogranite plutons of the Cumberland batholith are restricted to the upper structural level and indicate pre-emplacement imbrication of tectonostratigraphic units. Within both upper and lower plates, granulite facies assemblages are retrogressed to upper amphibolite assemblages in proximity to 1.82–1.79 Ga accretion–collision structures. Exposures of Superior Province basement across the orogen document a progressive increase in the extent of reworking during accretion of upper plate units, from a narrow shear zone in the external zone to km-scale imbrication and more pervasive shear deformation in the internal zone. A minimum of 160 km of basement underthrusting is constrained by the structural overlap of upper plate allochthons on lower plate units. Basement involvement during post-collision shortening is manifest as thick-skinned folding, indicating coupling across the collisional basal décollement.